Friction between surfaces is one of the largest causes for energy losses in mechanical appliances such as internal combustion engines and various machines and mechanisms. Friction also leads to wear which limits service lifetime of said appliances. Therefore, in many applications, there is a general request to provide surfaces having as low friction and experiencing as little wear as possible in contact with other surfaces. The most traditional way to reduce friction is by using lubricants. The lubricants keep the surfaces apart and are in themselves easily shearable, which reduces the force needed to achieve a relative motion. Liquid lubricants, such as oils are still the most used type of lubricants.
The utility of certain solid film lubricants has been known for quite some time. Here below are just a few examples presented. The U.S. Pat. No. 1,654,509 describes the use of graphite embedded into a metal binder to make an antiwear coating for bearings. The published patent application GB776502A describes protective films formed by treatment with vaporized reactive substances containing phosphorus, sulfur, selenium or halogen atoms. These protective films serve at least two functions advantageous to effective lubrication: (i) they provide a load-carrying film of solid lubricant; and (ii) they minimize carbonization and varnishing by inhibiting the catalytic activity of the metal. GB782263 shows that sulfurization of ferrous metal parts by heating the parts to a temperature above 500° C. in a fused salt bath containing alkali metal cyanide, alkali metal cyanate and active sulfur improves their resistance to wear and seizure. The published international patent application WO03091479A describes chemical treatment for piston rings and piston by heating in oil containing appropriate additives. The U.S. Pat. No. 5,363,821 discloses use of graphite, MoS2, BN solid lubricants incorporated into a polymeric carrier/binder for making antifriction coatings at the cylinder bore walls by spray-application with subsequent thermal fixation.
Still one method for production of a low friction coating is disclosed in the published Japanese patent application 2004-76914. A sliding member moves relative to a sliding surface and a lubricating oil containing molybdenum and sulfur is supplied to the sliding surface. Steel powder is added to a polyamideimide resin and by pressing the polyamideimide resin against the surface in the presence of said lubricating oil, molybdenum and sulfur are forced to react on the iron surface, yielding molybdenum disulfide held in a matrix of the resin. The resin thereby acts as a binder of the created molybdenum disulfide.
One could also mention a group of prior art techniques describing low-friction films produced by PVD, CVD and/or plasma-sputtering. Thus, the published US patent application 2005/0214540 describes PVD/CVD coatings for pistons, and the U.S. Pat. No. 4,629,547 describes low-friction boron-containing films obtained by plasma sputtering.
Common for most solid lubricant systems is that the lubricant is deposited onto the surface either as a pure lubricant substance or as a lubricant in a bearer substance. The deposition can be followed by different kinds of post treatments, typically thermal treatments or mechanical treatments. The lubricants will thus be provided as a layer on top of the surface to be lubricated. It is difficult to obtain a good adherence to the surface at the same time as a low friction is to be shown to a neighboring surface.
Advantages of tribofilms in reducing friction and wear are well known in the field of tribology. Tribochemical deposition of solid lubricants has advantageously been used in the published international patent application WO2009/071674, which discloses how to produce a tribochemically deposited film by mechanically rubbing a tool, typically comprising Mo or W, against a surface to be covered in the presence of sulfur. Such tribochemically deposited films present very attractive properties concerning smoothness, wear resistance and low friction. One important note is that the tribochemical process involves the substrate material as well, causing a gradual transformation between the substrate material and the solid lubricant. WS2 and MoS2 tribofilms allow increasing the lubricant film strength, and as a result, the wear resistance is increased. Furthermore, the surface integrity is improved and fatigue wear during and after running-in is reduced. The films also have a low boundary friction. The tribofilms also have a low boundary friction.
A minor drawback with the method presented in WO2009/071674 is that the tool surface, typically being made of an alloy comprising Mo and/or W, is consumed in the process and must be replaced at intervals. Furthermore, the tribofilm deposition rate is limited by relatively slow heterogeneous reaction between solid metal and active sulfur.